With the assumption that you have a two-way crossover of some sort, you would need two different crossovers--one for 4 ohms and one for 16 ohms. Let us assume that the woofer line is either 4 or 16 ohms. Then the impedance sensitivity would be different for these two impedances so the tweeter (or tweeter line) would necessitate separate crossovers or else their sensitivities wouldn't align across the frequency band.

Now if you just added a resistor into for a WWW/WRW/WWW configuration, two things are of concern. First, the series resistor would impact the box tuning for the center set of drivers. Second, the SPL output of this center set would be less than the outer two sets. Typically, you would prefer a constant output level from all drivers in the line or a taper of high SPL to lower at the ends of the line.

Jim